A polyacetal resin is an engineering plastic having a well-balanced mechanical strength and impact resistance and is widely used as automobile parts and mechanical structural components of electrical and electronic equipment.
The polyacetal resin has a high volume resistivity and surface resistivity and thus has an excellent electrically insulating property, but, because of its good electrically insulating characteristics, static electricity is easily generated, and therefore, the polyacetal resin has a problem in that it cannot be used for electrical parts which are likely to suffer electrostatic problems, or for dustproof parts.
Methods for imparting an antistatic property to a polyacetal resin by incorporating various antistatic agents into the polyacetal resin have been proposed. For example, as the antistatic agent having a relatively low molecular weight, an N,N-bis(alkoxy)amine (Japanese Examined Patent Publication No. 46-1857) and an ester of glycerol with a monofatty acid and boric acid (Japanese Unexamined Patent Publication No. 50-151952) are known.
As the antistatic agent having a high molecular weight, a polyether-esteramide block copolymer comprising a nylon and poly(tetramethylene oxide)glycol (Japanese Unexamined Patent Publication No. 59-191752) is known.
Moreover, there is known a method comprising incorporating a polyether ester block copolymer comprising polybutylene terephthalate and a polyoxolane having a relatively low molecular weight as the poly(alkylene oxide)glycol into a polyacetal resin (Japanese Unexamined Patent Publication No. 51-64560).
The low-molecular-weight antistatic agents disclosed in Japanese Examined Patent Publication No. 46-1857 and Japanese Unexamined Patent Publication No. 50-151952, however, have problems in that a bleeding of the antistatic agents occurs and the commercial value of products is reduced, and when the products are washed with water, the antistatic agents are removed by the water washing and the antistatic property is lowered.
The polyether-esteramide block copolymer comprising nylon 12 and poly(tetramethylene oxide)glycol, disclosed in Japanese Unexamined Patent Publication No. 59-191752, does not have a satisfactory antistatic property and satisfactory mechanical characteristics, because the antistatic property of the poly(tetramethylene oxide) chain is low, the compatibility between a polyacetal resin and the polyether-esteramide block copolymer is poor, and the size of dispersed particles is large.
The polyether-ester block copolymer comprising polybutylene terephthalate and a polyoxolane having a relatively low molecular weight, disclosed in Japanese Unexamined Patent Publication No. 51-64560, has a poor antistatic property and a poor compatibility with a polyacetal resin, as well as the above-mentioned polyether-esteramide block copolymer, and therefore, there is little improvement of the mechanical characteristics of the polyacetal resin and the antistatic property is not satisfactory. The same may be said of poly(alkylene oxide) chains other than the chain of (ethylene oxide) units, for example, poly(propylene oxide) chains.
Therefore, an object of the present invention is to provide a polyacetal resin composition having a permanent antistatic property and excellent mechanical characteristics.
The inventors carried out research with a view to solving the above problem, and as a result found that, if a polyether-ester block copolymer having a specific copolymerization composition, in which the polyether component is a poly(ethylene oxide)glycol residue having a specific molecular weight, is incorporated into a polyacetal resin, the above-mentioned object can be attained. The present invention is based on this finding.